Ynones with alkenes

Cycloadditions and cycUzadon. 2-Alken-7-ynones undergo a [2 + 2 + 2]cyclo-addition with electron-deficient alkenes to give cyclohexene derivatives. This process is complementary to the Diels-Alder reaction. 

Furfurylidenetrialkylphosphoranes. These Wittig reagents are generated from conjugated 2-alken-4-ynones by the addition of R,P. In situ reaction with carbonyl compounds leads to 2-alkenylfurans. 

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

There is a striking difference between the photochemical reactivity of oc,(3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo [2+2] cycloaddition to alkenes at the C=C double bond of the enone (probably from the triplet nn state) to yield cyclobutanes, acyclic enones easily deactivate radiationless by rotation about the central C-C single bond. Ynones on the other hand behave much more like alkyl-substituted carbonyl compounds and add to (sterically less encumberd) alkenes to yield oxetanes (Sch. 11) [38,39]. The regioselectivity of the Paterno-Biichi reaction is similar to that of aliphatic or aromatic carbonyl compounds with a preference for primary attack at the less substituted carbon atom (e.g., 41 and 42 from the reaction of but-3-in-2-one 40 with... 

Preliminary investigations describing the photoadditions of ynones to alkenes have determined that these species may react through n,Tt excitation to produce aUcynic oxetanes. Irradiation of 3-butyn-2-one with isobutene gave a 46% isolated yield of (72) and (74) (14 86). 3-Octyn-2-one analogously provided a 50% isolated yield of (73) and (75) (15 85), a regioselectivity predicted on the basis of the best diradical intermediate. 

Another illustration of the power of this methodology to construct natural products is illustrated by Scheffold s synthesis of PGF2Q, in the manner portrayed in Eq. (38) [46]. Here the initially formed radical undergoes a 5-exo,trig cyclization with the simple alkene found in 199. The resulting radical, when intercepted by ynone 200, affords enone 201 the latter is an advanced stage intermediate en route to PGF2 . 

C-Glycosidation of enol silane 279 to lactol acetate 278, prepared from 277 in two steps, furiushed ynone 280 as a single isomer. Reduction of the ketone with L-selectride furnished alcohol 270 with poor selectivity, but the minor isomer can be converted into the desired isomer via the Mitsunobu protocol Dihydroxylation of the terminal alkene, reduction of alkyne, and oxidative cleavage of the resulting triol gave the intermediate hydroxy aldehyde, which was spontaneously transformed into macrolactol 281 as a single diastereomer. 

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